The present invention relates to coated substrates, in particular transparent substrates coated with a stack of layers comprising one or more metal layers.
Such coated substrates are used, for example, as glazing units for buildings or vehicles, in single, double or laminated structures and as optical filters.
It is desirable that glazing units for buildings or vehicles do not allow too substantial a proportion of total incident solar radiation to pass through so as not to overheat the interior of the building or the passenger compartment, while providing a light transmission (LT=percentage of incident light flux—of illuminant D65—transmitted by a substrate) that is suitable in order to supply a sufficient level of illumination to the interior of the building. The transmission of the total incident solar radiation can be expressed in terms of the solar factor (SF=percentage of incident energy radiation, which, on the one hand, is directly transmitted by the substrate and, on the other hand, is absorbed by this then radiated by its opposite face to the energy source). These somewhat conflicting requirements express the requirement to obtain a glazing unit with an elevated selectivity (S), defined by the ratio of light transmission to the solar factor. It is also desirable that glazing units meet certain aesthetic criteria in terms of light reflection (LR=percentage of incident light flux—of illuminant D65—reflected by a substrate) and reflected colour.
The light absorption (LA) is the percentage of incident light flux (of illuminant D65) absorbed by a substrate, the sum of LT+LR+LA being equal to 100%.
The present description uses the definitions of the Internationale Commission on Illumination—“Commission Internationale de l'Edairage” (CIE).
Substrates bearing a coating with a high light transmission, low emissivity and reduced solar factor are known. The patent documents FR 2.641.271 and FR 2.641.272 of the Applicant describe a substrate of vitreous material bearing a multilayer coating, which comprises a reflective layer of silver sandwiched between two transparent and non-absorbent layers, one on the bottom and one on the top.
The patent document FR 2.733.495 of the Applicant describes sun protection stacks based on two layers of silver separated by non-absorbent layers, in particular of metal oxides. A fine sacrificial metal layer is deposited on each of the silver layers. This fine metal layer acts as a barrier to protect the silver layer from oxidation when the upper metal oxide layer is deposited. Its thickness is adjusted so that when the upper layer is deposited, it oxidises sufficiently to permit a maximum light transmission. In FR 2.733.495 the stack retains a high light transmission in the order of 76% on a 6 mm single clear glass substrate and a low absorption. The solar factor is in the order of 43% when the substrate is a 6 mm single clear glass sheet and in the order of 37% for a double glazing unit.
To further decrease the solar factor of the glazing a possibility could be to increase the thickness of the metal layer(s) of the stack or to increase the number of metal layers. However, this has the disadvantage of increasing the light reflection (LR) and of producing reflected colours in the glazing that are generally considered unpleasing.
U.S. Pat. No. 6,190,776 describes a heat treatable stack formed from a first dielectric layer/a first layer of Cr/a layer of Ag or Cu/a second layer of Cr/a second dielectric layer. This stack provides non-neutral reflected colours both before and after thermal treatment. For architectural applications with a low solar factor, the colorimetric index a* varies between 1.35 and 10.30 and index b* varies between −7.80 and 31.70, which gives purple to orange colourations. The light absorption of the glazing are around 23% after heat treatment.
Metallic barriers are also known that have a protective function during a subsequent thermal treatment. These barrier layers do not remain metallic, but oxidise during the thermal treatment and are no longer absorbent.
There are also sun protection stacks comprising absorbent layers. However, it is not obvious to add an absorbent layer without causing the light transmission to fall. Therefore, it is particularly difficult to maintain a significant selectivity while retaining a stack that is neutral (or aesthetically acceptable) in reflection.
EP 185 314 describes a stack comprising an absorbent layer, a layer of silver and a fine layer of stainless steel. The light reflection in this case is very high (up to 67%) and the LT is not more than 12%. These characteristics do not meet the desired criteria.
Stacks of the type Si3N4/NiCr/Ag/NiCr/Si3N4 are described in U.S. Pat. No. 5,800,933. Non-absorbent layers of Si3N4 can be doped with stainless steel. The nickel-chromium layers have thicknesses in the order of 0.7 to 0.8 nm. These stacks are adapted to obtain a light transmission of more than 70% and a low emissivity. The total absorption of such a stack on a clear glass is very low (in the order of 14% in a double glazing, LT=70%, LR=16%).
WO 02/48065 describes stacks with double layers of silver comprising an absorbent layer inserted directly between two dielectric layers. This absorbent layer must be placed either between the two layers of silver, “in” the intermediate dielectric, or “in” the upper dielectric. The solar factor attained is 26 to 31% in double glazing. The aim of these stacks is to preserve good optical properties after toughening. The insertion of an absorbent layer between two dielectric layers has the drawback to complicate the manufacturing process. Indeed, the deposition conditions of the dielectric layer (in non metallic condition) must be interrupted to deposit an absorbing layer in metallic conditions.
EP 1 032 543 describes a sun protection stack comprising a non-absorbent layer, a first infrared reflective layer, a sacrificial barrier, a second non-absorbent layer, a second infrared reflective layer, a second sacrificial barrier and a last non-absorbent layer. An absorbent layer is added under either one of the infrared reflective layers. Such stacks have a solar factor in the order of 31% (in double glazing).
However, there is a need to provide coated substrates, wherein the solar factor is still lower and the other optical characteristics remain favourable: low reflection from the coated side and from the non-coated side, high selectivity, aesthetically pleasing reflected colour etc.